In recent years, owing to the demands for higher integration degrees and higher operation speeds of LSIs, design rules for semiconductor devices that constitute LSIs are increasingly miniaturized. Along with this trend, the resistivity of transistor gate electrodes used in DRAMs, flash memories, and so forth is required to be lower. Conventionally, poly-silicon is used for gate electrodes, but entailing a disadvantage such that it increases the sheet resistance. Accordingly, it has been proposed to laminate a poly-silicon layer with a silicide layer of a refractory metal, such as tungsten, which is low in resistivity, high in adhesion to silicon, and good in processability. Specifically, for example, gate electrodes having a metal polycide structure, such as tungsten polycide (a laminated film of WSi/poly-Si) have attracted attention.
In general, where a transistor gate is set up, a well, a gate insulating film, and a gate electrode are formed in this order. When the gate electrode is formed, an etching process is performed. With this process, a side surface of a poly-silicon layer included in the gate electrode is exposed. When a voltage is applied to the gate electrode, an electric field is concentrated at this exposed portion and may cause malfunctions of the product, such as an increase in leakage current. Accordingly, it is necessary to perform an oxidation process to oxidize the exposed poly-silicon portion of the gate electrode, thereby forming an insulating film thereon. This oxidation of poly-silicon is conventionally performed by thermal oxidation. However, there has been proposed a plasma oxidation process utilizing plasma for the same purpose (for example, WO 2004/073073).
Where a poly-silicon layer is oxidized by plasma and the plasma oxidation conditions are suitably selected, the poly-silicon layer can be preferentially oxidized while oxidation of a metal-containing layer is being suppressed. Further, in this case, the poly-silicon layer is prevented from being bitten by an oxide film portion, which is called “birds-beak”, at its edge portions.
However, if formation of the birds-beak is suppressed too much when a poly-silicon layer is oxidized by plasma, the shape of the edge portions of the poly-silicon layer essentially unchanged with an acute angle. In this case, an electric field is concentrated at these portions and may increase leakage current. For this reason, the plasma oxidation conditions are reversely selected, so that oxidation is developed in a lateral direction at the edge portions of a poly-silicon layer to form a portion so called a birds-beak.
However, where a poly-silicon layer is oxidized by a plasma oxidation process to form a birds-beak at the edge portions of a gate electrode having a metal polycide structure, the process conditions need to be selected to provide a relatively strong oxidizing effect, which entails a decrease in the oxidation selectivity of poly-silicon relative to a metal in the metal silicide layer. Consequently, the metal in the metal silicide layer is oxidized, and brings about problems, such as a shape distortion due to expansion of the metal silicide layer or the like, and particle generation due to metal oxides, which drastically deteriorate the reliability of semiconductor devices.